Crescent Nebula (NGC 6888)

A rapidly expanding, luminous shell of gas that surrounds the Wolf-Rayet
star WR 136 in the constellation Cygnus.
The Crescent Nebula started to form about 250,000 years ago as the central
star began shedding its outer envelope in a strong stellar
wind. As this wind rammed into the surrounding interstellar
medium, it compacted it and sent shock
waves into it, resulting in a series of shells, which ultraviolet radiation
from WR 136 subsequently ionized and made visible.